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Family History Through the Alphabet – X is for X-Chromosomes and X-STR Tests

This week’s Family History Through the Alphabet Challenge features the letter X. Although I do not have any specific X’s in my ancestry (i.e., surnames, locations, etc.), noteworthy X’s with respect to family history certainly include X-Chromosomes and X-STR Tests.

X is for X-Chromosomes and X-STR Tests:

The use of DNA in genealogical research is a growing trend that many use to learn important information about their ancestry. There are several different types of test that can be used to accomplish this, which usually involve DNA from the Y-chromosome or Mitochondrial DNA (mtDNA). Another test, however, involves the X-chromosome.

An X-chromosome is one of two chromosomes that play a role in sex determination—the other being a Y-chromosome. Males have one X-chromosome and one Y-chromosome, while females have two X-chromosomes. Thus, the X-chromosome is universally found among males and females.

With respect to the X-chromosomes inherited by females, one comes from her mother, which is itself a mixture of her parents, and one comes from her father, which is itself a mixture of his parents. The X-chromosome inherited by males comes exclusively from their mother. It is thus, a mixture of the X-chromosomes she inherited from her mother and her father.

Generally, the use of X-chromosomes in genealogical research is problematic because in females the two X-chromosomes randomly swap information and genes (undergo recombination) during cell division (meiosis). Nevertheless, X-chromosomes can be used to study genealogical relationships, though the technology is newer and less widely used than the more traditional ones involving Y-chromosomes or Mitochondrial DNA (mtDNA). As with Y-chromosomes, X-chromosomes contain short tandem repeat (STR) markers, or X-STRs for short. Genealogy DNA tests that make use of X-chromosomes (X-STR Tests) do so by focusing on regions of X-STRs called “haplotype blocks,” which are inherited over several generations despite recombination during meiosis. FamilyTreeDNA was one of the first to use this method. According to their website, this ancestral DNA analysis “creates a unique inheritance pattern that while challenging to follow may provide many insights into one’s maternal heritage.” It should be noted, however, that an X-STR Test alone will not provide a full report, so the more traditional tests are more widely used.